Oscillation detection

ABSTRACT

The invention relates to oscillation detection and, more particularly, concerns a method and apparatus for identifying oscillation in a signal due to feedback, permitting appropriate action to be taken to suppress the oscillation.  
     The method involves using an FFT device or similar to convert a signal at each of a series of successive time windows into the frequency domain, calculating, for each of a plurality of frequency bands, the change in signal phase from a time window to a subsequent time window, and comparing, for some or all of said frequency bands, the results of the calculation step to one or more defined criteria to provide a measure of whether oscillation due to feedback is present in the signal. For additional discrimination, the change in signal amplitude from a time window to a subsequent time window may also be calculated for each of the frequency bands, and the result compared with one or more further defined criteria.  
     The invention has particular application in hearing aid devices.

FIELD OF THE INVENTION

[0001] The present invention relates to oscillation detection and, moreparticularly, concerns a method and apparatus for identifyingoscillation in a signal due to feedback. The present invention may beused in conjunction with the method and apparatus for suppressingoscillation in a signal described in applicant's copending applicationentitled ‘Oscillation Suppression’ (Attorney ref. 30-518-9119).

BACKGROUND OF THE INVENTION

[0002] In this specification, where a document, act or item of knowledgeis referred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date:

[0003] (i) part of common general knowledge; or

[0004] (ii) known to be relevant to an attempt to solve any problem withwhich this specification is concerned.

[0005] Acoustic amplifiers are used in many common applications such astelephones, radios, headsets, hearing aids, and public address systems.Typically, such an application comprises a microphone or other inputtransducer to pick up sounds and convert them into an electrical signal,an electronic amplifier to increase the power of the electrical signal,and a speaker or other output transducer to convert the amplifiedelectrical signal back into sound.

[0006] If the input and output transducers are close enough, the outputacoustic signal may be picked up by the input transducer and fed backinto the amplifier with a delay, the delay being the time taken for thesound to travel from the output transducer to the input transducer (plusany delay due to the electrical processing of the signal). This is‘acoustic feedback’. Electrical feedback can also occur if theelectrical signal at the output is coupled back to the input, forexample by inductive or capacitive coupling. Further, mechanicalfeedback can also occur if vibrations are transmitted from the outputtransducer to the input transducer via the body or case of theamplification system. Under feedback conditions, the device can thenbecome unstable and the components begin to ring. The ringing thenself-reinforces and increases in intensity to drive the components intosaturation. FIG. 1 illustrates a feedback loop, showing diagrammaticallythe components in an acoustic amplifier circuit, namely microphone 1,amplifier 2 and speaker 3, with feedback loop 4 representing the outputsignal feeding back to the input transducer.

[0007] All forms of feedback may result in instability or oscillation ofthe output signal from the amplifier under certain conditions.Oscillation and instability are undesirable because they distort thesignals being amplified and can result in very loud unpleasant sounds.In the case of hearing aids, this can lead to problems both for thewearer and for those around. The conditions for oscillation are that thetotal gain around the loop must be greater than 1, so that the signal isfed back into the system with a greater intensity each time, and thetotal delay around the loop must be a whole number of periods of theoscillation frequency, so that the input and output signals addconstructively. Equivalently, the total phase change around the loopmust be a multiple of 2π radians for the oscillation frequency. Thesecriteria are set out in equations 1 to 3 below.

Loop Gain>1  (eq. 1)

Loop Delay=N×period  (eq. 2)

Loop Phase Change=2Nπ radians  (eq. 3)

[0008] (where N is a positive integer)

[0009] Any electronic system containing a microphone and speaker inclose proximity may suffer from acoustic feedback. In hearing aids, thisoften results in the wearer experiencing unpleasant audible effects suchas loud whistling tones at certain frequencies, usually highfrequencies.

[0010] The traditional procedure for increasing the stability of ahearing aid is to reduce the gain at high frequencies, as suggested in,for example, U.S. Pat. No. 4,689,818. This may be done by setting themaximum gain value for each frequency, or automatic high frequency (HF)gain roll-off may be used. Controlling feedback by modifying the systemfrequency response, however, means that the desired high-frequencyresponse of the instrument must be sacrificed in order to maintainstability.

[0011] Efforts have been undertaken to reduce tie susceptibility ofhearing aids to feedback oscillation by improving the fit and insulatingproperties of the ear mould. Efforts have also been undertaken from anelectrical standpoint, from attenuation and notch filtering, asdisclosed in U.S. Pat. No. 4,088,835, to estimation and subtraction ofthe feedback signal, as disclosed in U.S. Pat. No. 5,016,280, tofrequency shifting or delaying the signal, as disclosed in U.S. Pat. No.5,091,952. Many different approaches to an electrical solution withcontinuous monitoring of the feedback path have been documented in therelevant literature.

[0012] A technique commonly used to suppress feedback in public addresssystems is a frequency shift, in which the input signal is altered by afew Hertz prior to being output at the receiver. This approach has notbeen particularly successful in hearing aids because a large frequencyshift is required to achieve a significant increase in gain. In hearingaids, the distance between microphone and receiver is much smaller thanin public address systems, and thus a feedback signal with only a smallfrequency shift may still be relatively closely in phase with the input.

[0013] Signal phase can also be altered by using a time-varying delay[1]. While this can provide 1-2 dB of additional useable gain, it canalso result in an audible ‘warbling’ effect. All pass filters have alsobeen used to modify the phase response of the feedback loop, but it canbe difficult to achieve satisfactory phase at all frequencies. Methodshave been proposed to push danger regions in the phase response tofrequencies outside the primary audio range where suppression can beapplied without loss of sound quality [2] [3]. These techniques stillassume that the feedback path is constant however, and no suggestion hasbeen made that an adaptive implementation may be developed.

[0014] The most common gain altering approaches attempt to reduce thesystem gain only in narrow bands where feedback is likely to occur. Thishas been attempted with a variety of notch filter implementations [1][4] [5]. Adaptive notch filtering has allowed 3-5 dB of additionaluseable gain. Two of the biggest problems with notch filteringtechniques have been the inability to accurately track the variations inthe feedback path with a narrow band, and the effects on normal spectralcontent with a broader band. In addition, the notch filter can actuallycontribute an additional phase change to the loop and shift thefrequency of oscillation as soon as it is applied.

[0015] Substantial increases in useable gain have been achieved byinserting an additional feedback path, based on an estimation of thereal feedback path, but 180 degrees out of phase. Early adaptiveimplementations of such systems performed continuous estimation of thefeedback path by inserting noise signals with appropriate statisticalproperties at the receiver and correlating the output with the input atthe microphone [1] [6]. These reported up to 10 dB of additional useablegain [7] but, since the noise ‘test’ signals were audible and unpleasantfor most wearers, this particular technique never became particularlywidespread.

[0016] More recent feedback cancellation systems of this type rely onsounds in the environment to perform their correlation [8]. To avoidartefacts and incorrect suppression of speech however, the estimationtime has to be longer than in systems using unnatural sounds to performcorrelation. This means that sudden changes in the feedback path canresult in several seconds of whistling before successful cancellationoccurs. If implemented in conjunction with another technique to handlesudden changes, this approach can allow at least 10 dB of additionaluseable gain [9]. The benefits and limitations of such systems arediscussed in [10].

[0017] Nearly all of the techniques discussed here require someknowledge of the frequency of oscillation. However, as a result of thenature of direct and multiple reflected acoustical paths betweenmicrophone and speaker (or the changing acoustic properties of theear/earmould/hearing aid coupling with regard to hearing aids) thefrequency of acoustic feedback is unpredictable and may extend over asubstantial portion of the audio frequency spectrum (between 20 and20,000 Hz). As a result, it is desirable to have a circuit that canquickly identify an oscillation and its frequency.

[0018] U.S. Pat. Nos. 4,232,192 and 4,079,199 propose systems using aphase locked loop (PLL) adapted to recognize an oscillation when itoccurs. As is known, however, when the input signal falls off, a PLLtends to become unstable and to drift. The result of the drift is anundesirable periodic, acoustic noise signal.

[0019] U.S. Pat. No. 4,845,757 describes another oscillation recognitioncircuit. This circuit detects oscillations by looking for long-lastingalternating voltages having relatively large amplitude and relativelyhigh frequency. This is problematic in many applications because itmeans that the signal may contain feedback oscillations for some timebefore they are identified by such a circuit.

[0020] There remains a need in the art to provide an improved or atleast an alternative way of detecting oscillations in a signal in areliable, effective and rapid manner, and to apply appropriatesuppression to the signal upon detection.

SUMMARY OF THE INVENTION

[0021] The invention provides, in accordance with a first aspect, amethod of identifying oscillation in a signal due to feedback, themethod comprising the steps of:

[0022] converting the signal at each of a series of successive timewindows into the frequency domain;

[0023] calculating for each of a plurality of frequency bands the changein signal phase from a time window to a subsequent time window; and

[0024] comparing, for some or all of said frequency bands, the resultsof the calculation step to one or more defined criteria to provide ameasure of whether oscillation due to feedback is present in the signal.

[0025] This affords a technique for automatically monitoring whether thechange in phase over time in a frequency band is sufficiently constantto indicate the presence of an oscillation in the signal. The successivetime windows represent time intervals selected for desired performance,and are preferably of 1-100 ms duration. The windows may be discrete, orsuccessive such windows may overlap.

[0026] Preferably, the method includes the step of further calculating,for each of the frequency bands, the change in signal amplitude from atime window to a subsequent time window, and comparing the result of thefurther calculation step to one or more further defined criteria, toprovide a further measure as to whether oscillation due to feedback ispresent in the signal. This provides an additional level ofdiscrimination.

[0027] The signal-conversion into the frequency domain may be carriedout by way of a Fast Fourier Transform technique.

[0028] In a preferred form, for each frequency band, for each timewindow, the signal phase from one or more previous time windows iscompared with that from the current time window to calculate a change ofphase, and this phase change is then compared with a previous phasechange to provide a measure of the change in phase change.

[0029] Preferably, the signal phase change is calculated from each timewindow to the next successive time window, to provide a continuousmonitoring of the change in phase change in that frequency band.Alternatively, other approaches may be employed to monitor the phasechange over successive time windows, such as a statistical samplingtechnique.

[0030] A counter may be employed, the counter incremented if the valueof the change in phase change is within a prescribed limit, the counterbeing reset if it is not, the measure of whether oscillation due tofeedback is present in the signal being provided by the counter reachinga value M_(p).

[0031] If signal amplitude monitoring is employed, the method mayinclude the step of, for each frequency band, for each time window,comparing the amplitude from at least a previous window with that of thecurrent window to calculate a change in amplitude.

[0032] A counter may be employed, the counter being incremented if thevalue of the amplitude change is greater than zero, the counter beingreset if it is not, the further measure of whether oscillation due tofeedback is present in the signal being provided by the counter reachinga value M_(a).

[0033] The value of M_(p) and/or M_(a) is selected as appropriate,dependent on the specific application and the level of sensitivityrequired to achieve the desired performance.

[0034] In one form of the invention, M_(p) is equal to M_(a).

[0035] Preferably, on determination that oscillation due to feedback ispresent in the signal, a selected 20 method for suppressing oscillationis applied to the signal in that frequency band.

[0036] The suppression technique employed may include the step of addinga random phase to the signal in at least one of said frequency bands fora prescribed period of time. Alternatively, the suppression techniquemay be selected from the group of: applying a phase shift; applying anotch filter; subtracting a signal from the input signal; and applying again attenuation.

[0037] The invention provides, in accordance with a second aspect,apparatus for identifying oscillation in a signal in a system having aninput transducer and an output transducer, comprising:

[0038] means for converting the signal into the frequency domain;

[0039] means for analysing the converted signal at each of a successionof time windows over a number of frequency bands, to determine theamplitude and phase of the signal in each frequency band;

[0040] means for calculating the change in signal phase for eachfrequency band from a time window to a subsequent time window; and

[0041] means for comparing the change in phase with one or more definedcriteria to provide a measure of whether oscillation is present in thesignal.

[0042] Preferably, means are included for further calculating, for eachof the frequency bands, the change in signal amplitude from one timewindow to a subsequent time window, and means for comparing the resultof the further calculation step to one or more further defined criteria,to provide a further measure as to whether oscillation is present in thesignal.

[0043] The converting means may comprise a Fast Fourier Transform (FFT)unit.

[0044] The apparatus may include means for comparing, for each frequencyband and for each time window, the signal phase from one or moreprevious time windows with that from the current window to calculate achange of phase, and means for comparing this phase change with aprevious phase change to provide a measure of the change in phasechange.

[0045] Preferably, the means for comparing is arranged to calculate thesignal phase change from each time window to the next successive timewindow, to provide continuous monitoring of the change in phase changein that frequency band.

[0046] In one form of the invention, a counter is included, arranged tobe incremented if the value of the change in phase change is within aprescribed limit, and to be reset if it is not, the measure of whetheroscillation is present in the signal being provided by the counterreaching a value M_(p).

[0047] If means are included for calculating the change in signalamplitude from one time window to a subsequent time window, this maycomprise means for comparing, for each frequency band and for each timewindow, the amplitude from at least one previous window with that of thecurrent window, to calculate a change in amplitude.

[0048] A counter may be arranged to be incremented if the value of theamplitude change is greater than zero, and to be reset if it is not, thefurther measure of whether oscillation is present in the signal beingprovided by the counter reaching a value M_(a).

[0049] In a preferred form, the apparatus is provided in combinationwith a means for suppressing oscillation, the suppressing means arrangedto be triggered in accordance with the measure of whether oscillation ispresent in the signal.

[0050] The apparatus may include means for reconverting the signal to awaveform signal to be fed to the output transducer.

[0051] The invention differs from previous techniques because it relieson continuous monitoring of signal phase information as the primarycriterion for oscillation detection, thus allowing oscillationconditions to be identified before the amplitude of the signal at aparticular frequency becomes unstably high, ideally before audibleringing occurs.

[0052] The present invention therefore provides a feedback detectionsystem that continually monitors an input signal and recognises thepresence of an oscillation quickly and accurately. Further, theinvention provides alteration of the feedback loop in a manner thatdisrupts the feedback oscillation conditions and suppresses theoscillation without significantly affecting the system frequencyresponse.

[0053] In the preferred method of carrying out the invention, shortsamples or windows of the input signal are analysed into a number offrequency bands via a Fast Fourier Transform (FFT), the amplitude andphase of each frequency component is calculated and then checked againstthe following oscillation criteria:

[0054] 1. The change in phase from one window to the next must beconstant within an acceptable small variation for at least M_(p)successive windows.

[0055] 2. (Optional) The amplitude of the frequency component should beincreasing from one window to the next for at least M_(a) successivewindows.

[0056] The invention is based on the realisation that if an oscillationis present in a frequency band it will either dominate the band or beattenuated by destructive interference. Thus any band containing anoscillation that is not attenuated will have a reasonably constantchange in phase from one window to the next. In addition, any band thatis feeding back to the input will experience an increase in amplitude.By monitoring each frequency band with regards to at least the first ofthese criteria, the technique can be used to identify oscillation, oftenbefore the amplitude becomes uncomfortably loud. In addition, by usingthese two criteria in conjunction the system can avoid misdiagnosingloud sounds or most oscillating musical tones as feedback.

[0057] It should be noted that the feedback detection method may be usedwith any suitable approach to feedback suppression.

BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The present invention will become more apparent by describing indetail a preferred non limiting embodiment with reference to theattached drawings, in which:

[0059]FIG. 1 is a block diagram schematically illustrating a feedbackloop;

[0060]FIG. 2 is a block diagram of an apparatus according to the presentinvention;

[0061]FIG. 3 is a flow diagram illustrating the logic and process offeedback detection;

[0062]FIG. 4 is a flow diagram illustrating the logic and process offeedback suppression; and

[0063]FIGS. 5 and 6 are block diagrams of alternative architectures ofapparatus according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0064] An acoustic system 10 in accordance with the invention, such as ahearing aid, is schematically depicted in FIG. 2. A microphone 11converts an acoustic signal, such as the speech, into an analogueelectrical signal proportional to the acoustic signal, which signal isthen converted by an A/D converter 12 into a digital signal. The outputof A/D converter 12 is connected to the input of a Discrete FourierTransform (DFT) unit—such as a Fast Fourier Transform (FFT) unit 13—foranalysing the frequency components of the signal, and unit 14 enablesanalysis of 64 frequency bands across the spectrum of the signal. Asuitable unit is the Toccata Plus integrated circuit designed anddeveloped by the Dspfactory, operating with 16 kHz sampling rate andusing 128 point windows of 8 millisecond duration with 50% overlap toyield 64 linearly spaced frequency bands at 125 Hz intervals from 0 to8000 Hz. Module 20 is a feedback detector arranged to monitor the phaseand amplitude of the signal in each frequency band in the spectrum(adjusted if appropriate, as explained further below) during successivesampling windows at short intervals, such as successive 8 millisecondwindows with 50% overlap, calculated every 4 milliseconds. The apparatusincludes a counter for each frequency band, which can be incremented orreset at each successive time window.

[0065] For each time window, the measured phase from the previous windowis subtracted from the phase in the current window to calculate thechange in phase at a particular frequency band. This change in phase iscompared to the previous change in phase. If the values are within adefined variation (ie the change in the phase change is within thethreshold) then the counter is incremented, otherwise the counter isreset. Further, the amplitude in the current window is compared with theamplitude in the previous window. If the current amplitude is less thanthe previous amplitude, then the counter is reset. The feedback detectoris programmed to respond—by triggering feedback suppression—to thecounter reaching a value M. The present invention contemplates thateither the change in phase change criterion (counter reaches M_(p)) orthe change in amplitude criterion (counter reaches M_(a)) may beconsidered for suppression triggering, or both.

[0066] The example represented in FIG. 3 illustrates, for a time window,the process of detection using the change in phase change criterion. Foreach of the 64 bands, the state of the band is determined (30). If thatband is already being suppressed (31), no calculations are performed.Otherwise, the phase is calculated (32), and the previous phase valuecalculated for that band (which value has been stored—see below) issubtracted from the current phase value (33) to provide a current valueof phase change. The next step (34) is to subtract the previous phasechange value from the current phase change value, to output a value ofchange of phase change. This value is then checked (35) and (37), and ifit is within a prescribed threshold for phase change variation, thecounter is imcremented by 1 (41). The subtraction of 2π radians (36) andsecond check (37) ensure that the value of the change of phase change ischecked, irrespective of whether the change has increased or decreased.If the value is not within the threshold, the counter is reset to 0(38), the current phase and phase change value is saved (39), and thenext band is selected (40).

[0067] If the counter has been incremented (41), a check is made todetermine if it has reached a value M_(p) (42), thereby indicating anoscillation has been detected (43) and flagging that band forsuppression (see below). If not, the current phase and phase changevalues are saved (39), and the next band is selected (40). It is to benoted that the bands can be checked in parallel or sequentially withineach time window.

[0068] In simulations carried out by the inventors, where both criteriafor detection have been employed, M_(a)=M_(p)=12 gives good performance.Using M_(a)=M_(p) simplifies the detection apparatus and method, as acommon counter can be used. If only one criterion is to be employed indetecting feedback, the M_(a) or M_(p) value may be increased to avoidfalse triggering of feedback suppression.

[0069] Once the counter for any frequency band exceeds the requiredvalues of M_(a) and/or M_(p), this frequency band is deemed to be inoscillation, and an apply phase module 21 is triggered (see FIG. 2).Apply phase module 21 generates a complex number with random phase andamplitude 1 for each window, and multiplies the real gain value atmodule 22 for the frequency band by this complex number before the gainis applied to the signal via gain unit 23 to provide an adjustedspectrum 24. The loop illustrated in FIG. 2 indicates that the phase ofthe gain multipliers depends on the apply phase unit, which depends onthe feedback detector unit. Apply phase module 21 continues to applyrandom phase to the gain for about one second, to allow the conditionswhich created the feedback path to change.

[0070] The example represented in FIG. 4 illustrates the process ofsuppression for a time window, appropriate for the example embodimentsillustrated in FIGS. 5 and 6. Firstly, the state of a selected band ischecked (40), to determine whether it is flagged for suppression (41).If not, the next band is selected (47). If it is flagged forsuppression, the magnitude of the signal at that band is obtained (42)and multiplied by the real part of the generated random complex number(43), the resulting new real component being saved (44). Further, themagnitude of the signal is multiplied by the corresponding imaginarypart of the generated random complex number (45), and the resulting newimaginary component saved (46).

[0071] The signal passes through MPO unit (Maximum Power Output) 25 (seeFIG. 2), and is then reconverted into a time domain waveform by inverseFFT module 26. A D/A converter 27 then converts the digital signal to anelectrical analogue signal before supplying it to the hearing aid outputterminal to drive speaker 28.

[0072] It is to be noted that the ‘magnitude of the signal’ in a bandreferred to above in the context of FIG. 4 may be the output spectrumvalue (for the example embodiments shown in FIGS. 5 and 6), or may bethe gain value (for the example embodiment shown in FIG. 2), and theinvention may be implemented using either approach, the selectiondepending at least in part on the hardware employed for the processing.In the alternative architectures of FIGS. 5 and 6 the random phase isapplied to the output spectrum rather than to the gains, in bothembodiments the gain values are applied to the signal by gain unit 23before feedback detector 20. In FIG. 6, MPO unit 25 is omitted, toillustrate that the invention can be implemented without it.

[0073] Feedback detector 20 and apply phase module 21 do not necessarilyhave to be applied together. An alternative form of feedbacksuppression, such as application of a notch filter, may be applied to asignal in which feedback oscillation has been identified by feedbackdetector 20. Other types of feedback suppression which might be employedinclude gain attenuation at the frequency band in question, applying atime varying phase change, or subtraction of the signal at the frequencyband in question. Similarly, an alternative form of feedback detector,such as a phase locked loop (PLL) circuit, may be employed, apply phasemodule 21 being used to apply a random phase to the signal in thatparticular frequency band once feedback has been detected.

[0074] It has been found in simulations carried out by the inventorsthat application of both feedback detector 20, combining the monitoringof both phase change and amplitude, along with the application of applyphase module 21, can result in suppression of all feedback oscillationin 60-100 milliseconds.

[0075] Modifications and improvements to the invention will be readilyapparent to those skilled in the art. Such modifications andimprovements are intended to be within the scope of this 20 invention.For example, in accordance with the invention, the signal spectrum maybe split into a plurality of discrete frequency bands, or alternativelyneighbouring bands may overlap. The word ‘comprising’ and forms of theword ‘comprising’ as used in this description and in the claims does notlimit the invention claimed to exclude any variants or additions.

1. A method of identifying oscillation in a signal due to feedback, themethod comprising the steps of: converting the signal at each of aseries of successive time windows into the frequency domain; calculatingfor each of a plurality of frequency bands the change in signal phasefrom a time window to a subsequent time window; and comparing, for someor all of said frequency bands, the results of the calculation step toone or more defined criteria to provide a measure of whether oscillationdue to feedback is present in the signal.
 2. The method of claim 1,including the step of further calculating, for each of the frequencybands, the change in signal amplitude from a time window to a subsequenttime window, and comparing the result of the further calculation step toone or more further defined criteria, to provide a further measure as towhether oscillation due to feedback is present in the signal.
 3. Themethod of claim 1, in which the step of signal conversion into thefrequency domain is carried out by way of a Fast Fourier Transformtechnique.
 4. The method of claim 1, in which the number of frequencybands is around
 64. 5. The method of claim 1, in which said successivetime windows are in the range of 1 to 100 ms.
 6. The method of claim 1,in which for each frequency band, for each time window the signal phasefrom one or more previous time windows is compared with that from thecurrent window to calculate a change of phase, and this phase change isthen compared with a previous phase change to provide a measure of thechange in phase change.
 7. The method of claim 6, in which the signalphase change is calculated from each time window to the next successivetime window, to provide a continuous monitoring of the change in phasechange in that frequency band.
 8. The method of claim 6, in which acounter is employed, the counter being incremented if the value of thechange in phase change is within a prescribed limit, the counter beingreset if it is not, the measure of whether oscillation due to feedbackis present in the signal being provided by the counter reaching a valueM_(p).
 9. The method of claim 2, in which for each frequency band, foreach time window the amplitude from at least one previous window iscompared with that of the current window to calculate a change inamplitude.
 10. The method of claim 9, in which a counter is employed,the counter being incremented if the value of the amplitude change isgreater than zero, the counter being reset if it is not, the furthermeasure of whether oscillation due to feedback is present in the signalbeing provided by the counter reaching a value M_(a).
 11. The method ofclaim 10 in which a second counter is employed, said second counterbeing incremented if the value of the change in phase change is within aprescribed limit, the second counter being reset if it is not, themeasure of whether oscillation due to feedback is present in the signalbeing provided by the second counter reaching a value M_(p) and, whereinM_(p)=M_(a).
 12. The method of claim 1, in which, on determination thatoscillation due to feedback is present in the signal, a selected methodfor suppressing oscillation is applied to the signal in that frequencyband.
 13. The method of claim 12 in which the suppression techniqueincludes the step of adding a random phase to the signal in at least oneof said frequency bands for a prescribed period of time.
 14. The methodof claim 12 in which the suppression technique is selected from thegroup of: applying a phase shift; applying a notch filter; subtracting asignal from the input signal; and applying a gain attenuation. 15.Apparatus for identifying oscillation in a signal in a system having aninput transducer and an output transducer, comprising: means forconverting the signal into the frequency domain; means for analysing theconverted signal at each of a succession of time windows over a numberof frequency bands, to determine the amplitude and phase of the signalin each frequency band; means for calculating the change in signal phasefor each frequency band from a time window to a subsequent time window;and means for comparing the change in phase with one or more definedcriteria to provide a measure of whether oscillation is present in thesignal.
 16. The apparatus of claim 15, including means for furthercalculating, for each of the frequency bands, the change in signalamplitude from one time window to a subsequent time window, and meansfor comparing the result of the further calculation step to one or morefurther defined criteria, to provide a further measure as to whetheroscillation is present in the signal.
 17. The apparatus of claim 15,wherein the converting means comprises a Fast Fourier Transform (FFT)unit.
 18. The apparatus of claim 15, including means for comparing, foreach frequency band and for each time window, the signal phase from oneor more previous time windows with that from the current window tocalculate a change of phase, and means for comparing this phase changewith a previous phase change to provide a measure of the change in phasechange.
 19. The apparatus of claim 18, wherein said means for comparingis arranged to calculate the signal phase change from each time windowto the next successive time window, to provide continuous monitoring ofthe change in phase change in that frequency band.
 20. The apparatus ofclaim 18, including a counter arranged to be incremented if the value ofthe change in phase change is within a prescribed limit, and to be resetif it is not, the measure of whether oscillation is present in thesignal being provided by the counter reaching a value M_(p).
 21. Theapparatus of claim 16, in which the means for further calculatingcomprise means for comparing, for each frequency band and for each timewindow, the amplitude from at least one previous window with that of thecurrent window, to calculate a change in amplitude.
 22. The apparatus ofclaim 21, including a counter arranged to be incremented if the value ofthe amplitude change is greater than zero, and to be reset if it is not,the further measure of whether oscillation is present in the signalbeing provided by the counter reaching a value M_(a).
 23. The apparatusof claim 15, in combination with a means for suppressing oscillation,the suppressing means arranged to be triggered in accordance with themeasure of whether oscillation is present in the signal.
 24. Theapparatus of claim 15, including means for reconverting the signal to awaveform signal to be fed to the output transducer.